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Re: Misalignment, MIPS, and ip_hdr(skb)->version

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On Sat, Dec 10, 2016 at 11:18:14PM +0100, Dan Lüdtke wrote:
> 
> > On 8 Dec 2016, at 05:34, Daniel Kahn Gillmor <dkg@fifthhorseman.net> wrote:
> > 
> > On Wed 2016-12-07 19:30:34 -0500, Hannes Frederic Sowa wrote:
> >> Your custom protocol should be designed in a way you get an aligned ip
> >> header. Most protocols of the IETF follow this mantra and it is always
> >> possible to e.g. pad options so you end up on aligned boundaries for the
> >> next header.
> > 
> > fwiw, i'm not convinced that "most protocols of the IETF follow this
> > mantra".  we've had multiple discussions in different protocol groups
> > about shaving or bloating by a few bytes here or there in different
> > protocols, and i don't think anyone has brought up memory alignment as
> > an argument in any of the discussions i've followed.
> > 
> 
> If the trade-off is between 1 padding byte and 2 byte alignment versus
> 3 padding bytes and 4 byte alignment I would definitely opt for 3
> padding bytes. I know how that waste feels like to a protocol
> designer, but I think it is worth it. Maybe the padding/reserved will
> be useful some day for an additional feature.

Note, if you do do this (hint, I think it is a good idea), require that
these reserved/pad fields always set to 0 for now, so that no one puts
garbage in them and then if you later want to use them, it will be a
mess.

thanks,

greg k-h

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Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-07 19:40 +0100
  Re: Misalignment, MIPS, and ip_hdr(skb)->version Dave Taht <dave.taht@gmail.com> - 2016-12-07 19:50 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-07 20:00 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-07 21:00 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-08 01:40 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-08 01:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-08 23:30 +0100
              Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-09 00:20 +0100
              Re: Misalignment, MIPS, and ip_hdr(skb)->version Willy Tarreau <w@1wt.eu> - 2016-12-11 09:10 +0100
                Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-11 11:50 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 13:30 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-10 14:30 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 21:20 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 21:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-10 21:40 +0100
          RE: Misalignment, MIPS, and ip_hdr(skb)->version David Laight <David.Laight@ACULAB.COM> - 2016-12-12 17:20 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-12 17:40 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-07 20:00 +0100
  Re: Misalignment, MIPS, and ip_hdr(skb)->version Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-12-08 01:40 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version Daniel Kahn Gillmor <dkg@fifthhorseman.net> - 2016-12-08 06:10 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Jiri Benc <jbenc@redhat.com> - 2016-12-09 12:30 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Dan Lüdtke <mail@danrl.com> - 2016-12-10 23:30 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version Greg KH <gregkh@linuxfoundation.org> - 2016-12-11 08:20 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-11 16:00 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-11 16:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Andrew Lunn <andrew@lunn.ch> - 2016-12-11 16:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Willy Tarreau <w@1wt.eu> - 2016-12-11 17:50 +0100

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